The CA-model FLOW-S* for flow-type landslides: an introductory account

Several studies have recently been carried out, aiming at characterising flow-type landslides and suggesting approaches for planning either active or passive countermeasures. The physics of flow-type landslides includes various complexities of multiphase materials characterised by heterogeneities in time and space. In cases of rapid or extremely rapid phenomena, the prediction of flow movement, and of the likely inundated areas, is usually a severe task, and is therefore important to develop reliable strategies for hazard assessment. Modelling and simulation techniques can be a precious tool for risk assessment and mitigation. Different conceptual approaches, ranging from laboratory experiments to numerical schemes, have recently been proposed. Among them, some are specifically aimed at evaluating the susceptibility or the hazard posed by debris flows. A family of models for simulating flow-type landslides has recently been developed within the frame of a cooperation project between CNR-IRPI and University of Calabria. Such models belong to the class of Cellular Automata (CA) for Macroscopic Fluids. The early releases of the family were quite empirical and simplified, and ignored most of the physical characteristics of the type of landslides under consideration. In the recent releases, the models resulted to be quite robust, did not show problems of numerical instability, allowing to fully incorporate even very-dense DEMs. FLOW-S* is the most recent version of the mentioned CA-family, by far the most physically-constrained. It has been developed by referring to the well-known "equivalent-fluid" and "geotechnical" modelling approaches, by properly transposing their fundamentals into the discrete space-time framework of the macroscopic CA method. In the model, the material moves from a given cell to one of the neighbourhood driven by the gravitational acceleration along the local slope, and affected by dissipative and pressure terms. Momentum conservation is guaranteed, as well as a proper management of collisions; mass conservation depends on processes of entrainment, which may occur along the flow path. Energy conservation depends on selected dissipative processes. Finally, model parameters reflect the CA approach, but are also related to the characteristics of the material involved, and to the type and rheology of the phenomenon. Model performances have been tested against several ideal cases, and by also considering real events recently occurred in Southern Italy. Further tests are being performed by considering data from flume experiments, by employing different types of water-debris mixtures. First results of sensitivity analyses and calibration/validation experiments are encouraging and underline the potential of the model for susceptibility mapping and for hazard mitigation purposes. An introductory account on the employed approach is presented in this paper, with examples of analyses performed by considering an ideal case (a roughly bi-planar surface), and a real event (the Vallone Favagreca debris flow) triggered on 12 May 2001 near Scilla, in Calabria - Southern Italy.